1. Field of the Invention
This invention relates to a method for combined fabrication of Indium Gallium Arsenide (InGaAs)/Indium Phosphide (InP) avalanche photodiodes (APD""s) and p/intrinsic/n type conductivity (p-i-n) photodiodes for simultaneous receiving and tracking.
2. Related Art
InGaAs photodiodes have smaller bandgaps than silicon-based photodiodes and are therefore preferable for use in high speed optical data transmission, and high speed switching, applications. In devices that include free space optical communications, or any application that concurrently utilizes APD""s (for receiving high data rate optical information) and p-i-n photodiodes (where low data rates and low noise amplifiers do not necessitate an APD), one must use separate detector chips for the two different devices.
An APD requires extraordinary control of the epitaxial structure and of the placement of the P-N junction compared to a low noise p-i-n. An APD requires very high electric fields in order to create gain or amplification. Therefore, an APD cannot be directly fabricated in low bandgap InGaAs alone because the field necessary for gain will cause premature breakdown in the device.
As free space optical telecommunication systems become more prevalent, there is a greater need for combined APD and pin photodiode detectors. This is particular true in systems that may be reconfigured dynamically where beam steering is important. For example, a quadrant p-i-n detector may be used to steer an optical head towards a laser transmitter, and at the same time, an avalanche photodiode may be used for high data rate reception. Another application is that of laser rangefinding, where an avalanche photodiode may be used for the ranging task, and a p-i-n photodiode may be used for a slower communications channel.
What would be desirable, but has not heretofore been developed, is a method for fabricating APD and p-i-n photodiodes on the same chip.
It is an object of the present invention to provide an InGaAs detector having APD and p-i-n photodiodes fabricated on a single chip.
It is an additional object of the present invention to provide a method for fabricating an InGaAs detector having APD and p-i-n photodiodes on a single chip.
It is an additional object of the present invention to provide an InGaAs detector having APD and p-i-n photodiodes wherein the bias for each of the photodiodes can be independently controlled.
The present invention relates to an InGaAs detector with APD""s and p-i-n photodiodes on a single chip. The APD""s and the p-i-n photodiodes are fabricated using three separate diffusions for the formation of the p-n junction. First, the entire wafer is patterned using SiNx, and holes opened in the SiNx wherever the p-i-n diodes are to be fabricated. A deep diffusion through the APD gain layer, field control layer, speed-up layers and into the InGaAs absorption layer forms the p-i-n photodiode. A second SiNx diffusion barrier layer is deposited, and the avalanche photodiode pattern is etched in it. This APD is formed by two additional diffusions, but the diffusion depth is such that the p-n junction is formed in the undoped InP gain region. Contacts are made to the anodes and cathodes of the p-i-n and APD separately, in order to offer independent bias control for the two types of photodiodes to the user. In this manner an InGaAs device having any desired combination of APD""s and p-i-n photodiodes can be constructed.